1. Field of the Invention
This invention relates to method of and system for lean-controlling an air-fuel ratio in an electronically controlled engine, and more paticularly to improvements in method of and system for lean-controlling an air-fuel ratio in an electronically controlled engine, suitable for use in an engine for a motor vehicle, provided with an electronically controlled fuel injection device, wherein an air-fuel ratio is feedback-controlled to the lean side form the stoichiometric air-fuel ratio in response to an output from a lean sensor generating an output signal substantially proportional to the concentration of oxygen in the exhaust gas.
2. Description of the Prior Art
In an internal combustion engine, particularly, in an engine for a motor vehicle, provided with an emission control measure by use of a three-way catalyst, it is necessary to strictly hold an air-fuel ratio of the exhaust gas (hereinafter referred to as the "exhaust air-fuel ratio") flowing through the catalyst to the vicinity of the stoichiometric air-fuel ratio. In view of this, there has been put into practical use a method of feedback-controlling an air-fuel ratio to the stoichiometric air-fuel ratio in response to a rich or lean signal outputted from an oxygen concentration sensor (hereinafter referred to as an "O.sub.2 sensor") generating a voltage in ON-OFF manner in accordance with a rich or lean condition of the exhaust air-fuel ratio with respect to the stoichiometric air-fuel ratio sensed from the concentration of oxygen in the exhaust gas. The above-described method of controlling a air-fuel ratio features that the air-fuel ratio can be feedback-controlled to the vicinity of the stoichiometric air-fuel ratio, so that the exhaust gas purifying performance by the three-way catalyst provided in an exhaust system can be satisfactorily improved. However, since the air-fuel ratio is constantly controlled to the vicinity of the stoichiometric air-fuel ratio in the above-described method of controlling the air-fuel ratio, the stoichiometric air-fuel ratio is maintained even in the operating condition where an air-fuel ratio to the lean side from the stoichiometric air-fuel ratio (hereinafter referred to as a "lean air-fuel ratio") is practically adoptable, such as in a light engine load region, whereby there have been some cases where the fuel consumption performance cannot be satisfactorily improved.
To obviate the above-described disadvantage, there has heretofore been attempted that the air-fuel ratio is brought to the lean side from the stoichiometric air-fuel ratio to effect a so-called lean combustion, so that the fuel consumption performance of the engine can be improved. This air-fuel ratio lean control method utilizes such a fact that a good correlation is observed between the concentration of oxygen in the exhaust gas and the air-fuel ratio when the lean air-fuel ratio is adopted, so that the air-fuel ratio in the exhaust gas can be continuously detected by measuring the concentration of oxygen in the exhaust gas.
As shown in FIG. 1, one of the sensors capable of measuring the concentration of oxygen in the exhaust gas and generating an output signal substantially proportional to the concentration of of oxygen (hereinafter referred to as a "lean sensor") includes:
a bottomed cylinder-shaped element body 10A made of an oxygen ion conductive, stabilized zirconia solid electrolyte;
an air-permeable measuring electrode (cathode) 10B provided on the outer surface of the element body 10A, made of a heat-resistant, electronically conductive body such as platinum and capable of introducing the exhaust gas as being the gas to be measured;
a diffusion-resistant layer 10C provided to coat the cathode 10B and formed into a porous ceramic material made of a heat-resistant inorganic substance such as alumina, magnesia or spinel for controlling the diffusion of the concentration of oxygen in the exhaust gas;
an air-permeable electrode (anode) 10D provided on the inner surface of the element body 10A, made of a heat-resistant, electronically conductive body such as platinum and capable of introducting atmosphere having a known concentration of oxyen (about 21%);
an atmosphere intake pipe 10E for taking in atmosphere along the anode 10D; and
a heater 10F provided in a gap of the atmosphere intake pipe 10E in such a manner that the forward end thereof approaches the bottom portion of the element body 10A, for heating the forward end portion (the bottom portion) of the element body 10A to a predetermined temperature, e.g., 650.degree.-700.degree. C. or more so as to make the element body 10A to function as an oxygen pump.
If current is passed between the aforesaid electrodes 10B and 10D in the above-described lean sensor 10, then oxygen can be moved in one direction through the electrolyte. However, the cathode 10B is coated by the diffusion-resistant layer 10C having pores for sending in oxygen smaller in value than an oxygen delivering capacity of the cathode 10B, so that the value of current can be held at a predetermined one in some applied voltage region. This predetermined current value is a so-called threshold current value. This threshold current value is varied substantially rectilinearly in proportion to the concentration of oxygen, so that, for example, the concentration of oxygen can be continuously detected from a variation in an output voltage from the lean sensor in which the threshold current value is converted to a voltage signal.
The air-fuel ratio lean control using the aforesaid lean sensor features that the air-fuel ratio can be feedback-controlled to the lean side from the stoichiometric air-fuel ratio. However, heretofore, when it is desired to make the air-fuel ratio different from a target air-fuel ratio (hereinafter referred to a "base air-fuel ratio") during normal operation condition in both the aforesaid control of the air-fuel ratio using an O.sub.2 sensor and the aforesaid air-fuel ratio lean control using the lean sensor, as in a warm-up fuel amount increase effected depending on an engine cooling water temperature, etc. during cold engine state for example and it is desired to vary the target air-fuel ratio to the rich side from the normal target air-fuel ratio, the feedback control has not been able to continue, and consequently, the feedback control has been stopped and an open-loop control has been adopted. In consequence, when the fuel amount increase or decrease has become necessary as described above, there has been presented such a disadvantage that fluctuations and dispersion in the air-fuel ratio cannot be corrected. Particularly, the air-fuel ratio lean control using the lean sensor has presented the disadvantages that the air-fuel ratio reaches an overlean extent exceeding the limit of misfire, thus deteriorating the operating performance, or the air-fuel ratio is brought to the rich side from the base air-fuel ratio, whereby the air-fuel ratio is brought into between the base air-fuel ratio and the stoichiometric air-fuel ratio, thus deteriorating the exhaust gas purifying performance and the fuel consumption performance.